<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            width: 400px;
            height: 400px;
            margin:0 auto;
            background-color: #666;
            display: flex;
            justify-content: center;
            align-items: center;
            border-radius: 100px;
        }

        .box .childrenBox{
            width: 100px;
            height: 100px;
            background-color: #999;
        }

    </style>
</head>
<body>
    <div class="box" >
        <span class="childrenBox" ></span>
    </div>
</body>
</html>